SURFACTANT COUNTERION DYNAMICS IN PYRIDINIUM OCTYL PHOSPHATE LYOTROPIC MESOPHASES - A NUCLEAR-RELAXATION STUDY

At increasing water concentrations, the pyridinium octyl phosphate (OCTP) forms successively lamellar, hexagonal mesophases and isotropic micellar solutions. The conformational and dynamical properties of OCTP in the lyotropic mesophases have been investigated by multinuclear resonance and relaxation at several magnetic field strengths. The molecular and C-H bonds order parameters of the octyl phosphate anion have been obtained from the P-31 chemical shift anisotropy, C-13-P-31 and C-13-H-1 dipolar splittings. The order parameters of C-H bonds as well as the dynamics of the conformational changes have been derived from the fits of both the splittings and longitudinal relaxation rates, assuming a model of trans half-arrow-right-over-half-left gauche isomerizations about the C-C bonds. The probabilities of the trans rotamer along the chain are found to be very high, exceeding generally 0.8. No significant differences are observed between the H(I) and L-alpha phases. In both phases the octyl phosphate anion behaves as a quite rigid molecule undergoing a very anisotropic reorientation with 10 < D parallel-to/D perpendicular-to < 20. Similar experiments have been done on a perdeuterated octanoic probe diluted in the mesophases. The H-2 longitudinal relaxation rates being independent of the frequency between 13 and 46 MHz, it is concluded that there is no significant contribution of collective motions to the longitudinal relaxation under our experimental conditions. The P-31 relaxation enhanced by the pyridinium protons shows that this cation and the octyl phosphate form a tight ion pair diffusing at about the same rate in the lamellar phase. In both phases, the pyridinium ion keeps however a large reorientational freedom evidenced by short correlation times and small order parameters.